Lots of foods such as fruits, vegetables and whole grains, among others, are naturally fat-free (or only have tiny amounts). But these aren't the ones that we're concerned about. Let's talk instead about the foods that have a significant amount of fat to begin with that we want to alter.
Dairy products are a good place to start. When we first get milk, it has a good bit of fat in it. For many, that's unwanted fat. How do we get from whole milk down to skim? The process is pretty simple. Manufacturers put the whole milk into a centrifuge that separates out the heavy fat portion into cream and leaves behind skim milk. This skim milk can then be used to make other dairy products like yogurt and sour cream that are either low-fat or nonfat. However, many of these nonfat dairy products that are made simply from skim milk run into the problems we mentioned before: They don't taste good, their textures are funny, and they may not last as long in our refrigerators [source: Millstone]. Dairy products in this category, like fat-free cheese, need to be made the way many diet products are created: not by taking fat out, but by never putting the fat in to begin with.
Instead of making a food product – let's say a cookie – and sucking the fat out (leaving behind something that is no longer anything like a cookie), food scientists need to create that cookie from the start using nonfat-based additives instead of fat to compensate for all that fat brings to the cookie. Let's start with flavor. Fatty cookie ingredients like butter and eggs add a lot of flavor. To make up for the lack of flavor when those items aren't used, manufacturers add in extra spices and a lot more sugar to trick us into not noticing the missing fat. This is why low- and nonfat foods aren't always less caloric, as their "diet" name might imply; they often have a ton of extra sugar added [source: Millstone]. To deal with texture issues, manufacturers will add food binders like gums and starches and also water. However, as a result of adding water, these fat-free products aren't as shelf stable, so surfactants and emulsifiers like mono- and diglycerides (close cousins to fats, which are triglycerides) must also be added.
In the past, food manufacturers have experimented with fat replacements like olestra, but have yet to find much success in their use. The side effects of these materials (let's just leave it at the phrase: anal leakage) resulted in very poor sales [source: Center for Science in the Public Interest].
Long story short: Getting the fat out of fat-free foods, for the most part, actually means never putting the fat in there to begin with. What goes in the place of fat varies, but you'll often end up with a highly processed food product that may be quite different from its fatty counterpart.